Aside of being parodies of one anime (various in the case of Carnival Phantasm), both share the same kind of humor and they will give you a similar feeling while watching them. So, if you liked one, you should try the other one.
Both are a lighter take on their respective universes. Though where Carnival Phantasm shines, Kira fails miserably. Regardless, the two are very similar in their intention. It is also highly recommended to watch the anime each OVA is based off, or you will not understand the jokes.
As thanks for playing in The World, its makers are pleased to announce a new hot springs location for the users! But like many special events, the area is hidden, and only those who can solve the mystery will find it. Tsukasa, Kite, Mimiru, and the rest of the .hack//SIGN and game world bunch are up for this challenge -- with one problem! The only clue lies in the word "Gift"...
Both of these parody anime feature an all-star cast of fantasy characters united from different series or franchises. If you've seen or read any of Fate Stay Night, Tsukihime or Melty Blood, AND any of the .hack series, plus enjoy comedy, I suggest watching the other.